At the time of launch, Air Force officials offered few details about the mission, saying that the space plane simply provided a way to test new technologies in space, such as satellite sensors and other components.

It was set to land on a 15,000-foot airstrip at Vandenberg Air Force Base, northwest of Santa Barbara. But the Air Force has never announced an exact landing date.

Although the X-37B program is classified, some of the particulars are known.

More than 10 years ago, it began as a NASA program to test new technologies for the space shuttle. But when the government decided to retire the aging fleet of shuttles, the Pentagon took over the program and cloaked it in secrecy.

Two X-37B vehicles were built by Boeing Co. in Huntington Beach. Engineering work was done at the company's facilities in Huntington Beach and Seal Beach. Components also came from Boeing's satellite-making plant in El Segundo.

The spacecraft is 29 feet long and has a wingspan of 15 feet. It draws solar power from unfolding panels.

This is the third time that the Air Force has sent an X-37B into orbit.

The first X-37B was launched in April 2010 and landed 224 days later at Vandenberg Air Force Base, northwest of Santa Barbara. The second X-37B spent 469 days in space.

Some industry analysts have theorized that because of the program’s clandestine nature, the X-37B could be a precursor to an orbiting weapon, capable of dropping bombs or disabling foreign satellites as it circles the globe.

The Pentagon has repeatedly said the space plane is simply a "test bed" for other technologies.

Brian Weeden, a former Air Force officer and expert in space security at the Secure World Foundation, said the X-37B is most likely testing new sensor technologies and satellite hardware. It may even be performing some surveillance over the Middle East region.